The present invention relates to a method of attaching an ultrasonic transducer to surface. It is frequently necessary to bond ultrasonic transducers to surfaces in an environment in which the bonding material will be subject to high temperatures and radiation. One example is the ultrasonic transducers which are used to monitor the vibration of a core barrel in a nuclear reactor. The bonding material in these situations may be subjected to temperatures as high as 600.degree.F and radiation levels as high as 50R (Roentgen) per hour. Investigations have shown that conventional, commercially available ultrasonic couplants are unsuitable for such conditions. For example, the commonly available colloidal grease type couplants will exhibit an excessive degree of thermal outgassing with eventual loss of physical properties and loss of mechanical bonding. Such changes would initially lead to constantly changing ultrasonic modulation which could generate erroneous ultrasonic data. Based on outgassing data, the in-service life of common high temperature couplants would be no greater than 700 hours at the temperatures encountered in nuclear reactor service. It is further estimated that under irradiation conditions, and with subsequent loss of adhesion and mechanical properites due to both the thermal and irradiation embrittlement, the useful service life of commercial couplants would be no greater than 50 hours. Because of this relatively short predicted service life, frequent changing of the couplant would be necessary. Radiation exposure levels as well as inaccessibility of the monitoring sites would make such a practice prohibitive.
One of the alternatives which has been considered to the use of colloidal grease type couplants is an alloy solder bonding system. A high temperature soldering system capable of withstanding 600.degree.F and 50R per hour without altering the modulation characteristics of ultrasonic pulse systems is not available. Furthermore, in-field application of such systems is highly cumbersome and, therefore, objectionable.
Thermosetting resin systems have also been considered for use as ultrasonic couplants since they possess the highest bond strength to weight ratio. However, such a bond would require extensive physical alteration to the bonding surface for adhesion efficiency. In most cases, such systems would not withstand temperatures in excess of 450F and would exhibit detectable monomer/cross-linking agent off-gassing for excessively long periods. Some of the thermosetting resin systems can be compounded to withstand temperature exposures approaching 600F but they require long post cure cycles which cannot be handled in in-field applications.
A great many of the commercially available ultrasonic couplants are not electrically conductive and it is desirable in nuclear applications that the couplant be conductive and serve as one of the electrical connections to the ultrasonic transducer. Some of these commercially available couplants can be rendered electrically conductive but the high conductive filler loads which are necessary greatly reduce the adhesion, the cross-linking efficiency and reduce the thermal stability. Ceramic adhesive systems have also been considered but they offer poor bonding strength. They are also highly inflexible and in most cases adequate bond strengths can only be developed by extended high temperature cure cycles at sintering temperatures. Such sintering cycles would have an adverse effect on the ultrasonic transducer and would make in-field applications impossible.